In order to achieve optimum joint clamp-up, it is desirable to optimize the preload of the fasteners used to clamp the joint. Fastener preload refers to the tension created in a fastener when it is first tightened. The preload of the fastener must be greater than the external forces applied to the joint in order to prevent joint gapping. Optimizing fastener preload allows structural joints to be designed that may reduce weight, and reduce or eliminate the need for retorquing fasteners over time in order to compensate for the affects of structural relaxation.
In the past, setting the tension in a fastener used to clamp a structural joint has been limited to controlling the amount of torque that is applied to the fasteners during installation, using a torque wrench or the like. However, the applied torque may not directly measure fastener preload and therefore may result in a relatively large margin of error. Moreover, once installed, it may be difficult to determine the amount of fastener preload and joint clamp-up, especially after the passage of time.
Accordingly, there is a need for a method and apparatus for directly determining fastener preload and monitoring joint clamp-up over time. It would also be desirable to provide a system for remotely monitoring fastener preload in order to periodically assess the health of structural joints.